Backlights are used to illuminate liquid crystal displays (“LCDs”). LCDs with backlights are used in small displays for cell phones and personal digital assistants (“PDAs”) as well as in large displays for computer monitors and televisions. Often, the light source for the backlight includes one or more cold cathode fluorescent lamps (“CCFLs”). The light source for the backlight can also be an incandescent light bulb, an electroluminescent panel (“ELP”), or one or more hot cathode fluorescent lamps (“HCFLs”).
The display industry is enthusiastically pursuing the use of LEDs as the light source in the backlight technology because CCFLs have many shortcomings: they do not easily ignite in cold temperatures, they require adequate idle time to ignite, and they require delicate handling. Moreover, LEDs generally have a higher ratio of light generated to power consumed than the other backlight sources. Because of this, displays with LED backlights can consume less power than other displays. LED backlighting has traditionally been used in small, inexpensive LCD panels. However, LED backlighting is becoming more common in large displays such as those used for computers and televisions. In large displays, multiple LEDs are required to provide adequate backlight for the LCD display.
Circuits for driving multiple LEDs in large displays are typically arranged with LEDs distributed in multiple strings. FIG. 1 shows an exemplary flat panel display 10 with a backlighting system having three independent strings of LEDs 1, 2 and 3. The first string of LEDs 1 includes seven LEDs 4, 5, 6, 7, 8, 9 and 11 discretely scattered across the display 10 and connected in series. The first string 1 is controlled by the drive circuit or driver 12. The second string 2 is controlled by the drive circuit 13 and the third string 3 is controlled by the drive circuit 14. The LEDs of the LED strings 1, 2 and 3 can be connected in series by wires, traces or other connecting elements.
FIG. 2 shows another exemplary flat panel display 20 with a backlighting system having three independent strings of LEDs 21, 22 and 23. In this embodiment, the strings 21, 22 and 23 are arranged in a vertical fashion. The three strings 21, 22 and 23 are parallel to each other. The first string 21 includes seven LEDs 24, 25, 26, 27, 28, 29 and 31 connected in series, and is controlled by the drive circuit, or driver, 32. The second string 22 is controlled by the drive circuit 33 and the third string 23 is controlled by the drive circuit 34. One of ordinary skill in the art will appreciate that the LED strings can also be arranged in a horizontal fashion or in another configuration.
There are many parameters in an LED string that can be controlled to optimize the efficiency or/and other operating targets of an LED string and driver, including temperature, luminous intensity, color, current and voltage. For example, current is an important feature for displays because the current in the LEDs controls the brightness or luminous intensity of the LEDs. The intensity of an LED, or luminosity, is a function of the current flowing through the LED. FIG. 3 shows a representative plot of luminous intensity as a function of forward current for an LED. As the current in the LED increases, the intensity of the light produced by the LED increases. The current in the LEDs must be sufficiently high to meet the desired brightness requirement. The drive current of the LED string is a function of the drive voltage applied to the LED string. In conventional displays, the drive voltage for the LED strings is fixed at a higher level than necessary, often with a large margin referred to as headroom, to ensure the operation of the LED strings under the worst case physical, electrical and ambient conditions and to account for the variations in the LEDs made by various manufacturers. That results in wastage of power.
Commercial electronic display systems are generally plugged into wall outlets, which provide around 110 volts alternating current (VAC) in the United States of America and around 220 VAC in some other countries. Some of the internal electrical components of the display systems operate with ac voltages and currents, for example, transformers. However, other internal electrical components of the display systems operate with direct current (dc) voltages and currents, for example, LED strings used for backlighting purposes.
To drive the LED strings, the conventional electronic display systems first convert the ac voltages and currents received from the wall outlets into dc voltages and currents by using a rectifier circuit. One of ordinary skill in the art will appreciate that the rectifier circuit can be a half wave rectifier or a full wave rectifier. Typically, the output of the rectifier circuit is further processed by a dc to dc converter. The dc to dc converter can be a switch regulator or a linear regulator. The dc to dc converter can be a part of a power factor correction circuitry. Next, the output of the dc to dc converter is scaled, typically by using another dc to dc converter, to obtain the desired drive voltage for the LED strings. It would be desirable to reduce the number of display system components by eliminating the dc to dc scaling converter.